In recent years, there have been developed internal combustion engines which is able to operate by the use of a plurality of kinds of fuels. In such internal combustion engines, there has been proposed a technique in which a gas fuel (CNG) is used during a period of time from engine starting until a catalyst for exhaust gas purification becomes active, and a liquid fuel (gasoline) is used after the exhaust gas purification catalyst has become active (see, for example, Patent Document 1).
In addition, there has also been proposed a technique in which a gas fuel (CNG) is used during the time when an internal combustion engine is in a cold state, and a liquid fuel (gasoline) is used after the internal combustion engine has been warmed up (see, for example, a Patent Document 2).